Conversion of carbonaceous materials to synthetic natural gas by reforming and methanation

ABSTRACT

A process for the production of synthetic natural gas from a carbonaceous material, preferably a biomass material, such as wood. The carbonaceous material is steam reformed to produce a syngas, which is then fed to a methanation zone to produce synthetic natural gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This is based on Provisional Application 60/832805 filed Jul. 24, 2006.

FIELD OF THE INVENTION

The present invention relates to the production of synthetic natural gas from a carbonaceous material, preferably a biomass material, such as wood. The carbonaceous material is steam reformed to produce a syngas, which is then passed through several clean-up steps then to a methanation zone to produce synthetic natural gas.

BACKGROUND OF THE INVENTION

The world's energy supplies, particularly liquid and gaseous fuel from fossil fuels, are being depleted faster than they are replaced. Consequently, the development of techniques for producing energy are urgently needed for avoiding the depletion of limited fossil fuel resources as well as for alleviating the global warming problem. Among various types of natural energy, biomass energy is regarded as one of the most promising natural energy from the viewpoint of its abundance, renewability and storability. Cellulosic materials, such as wood, have great potential for providing large amounts of energy. Direct combustion of woody biomass suffers from a limited amount of resource and low efficiency, and, further, only electric power can effectively be supplied from the direct combustion of woody biomass. The development of techniques that can utilize the entire biomass, including cellulose and hemicellulose, to produce energy, particularly in the form of liquid and gaseous fuels is of great interest. At the present time, however, such techniques are not in a practical stage for technical as well as economical reasons.

There is increasing interest in the production of synthetic natural gas as an alternative to natural gas. Synthetic natural gas, A large portion of synthetic natural gas is often referred to as “green gas” because it is a renewable gas typically obtained from biomass and having natural gas specifications. Thus, it can be transported through existing natural gas infrastructure, substituting for natural gas in all existing applications. Also, the use of biomass as the feedstock will not generally result in a net CO₂ emission as long as the source material can be replanted to replace those used as fuel. It may even be possible to reduce atmospheric CO₂ by sequestering the CO₂ that is released during the conversion of biomass (negative CO₂ emission).

Various problems exist in the art for pyrolyzing or gasifying carbonaceous materials, such as cellulosic materials. For example, vessels that have traditionally been used for gasifying biomass, such as wood chips and similar cellulosic material have been cylindrical, or often wider or narrower at the grate level than at the surface of the fuel bed, relative to the flow of feed and the forced air (or other gases) draft. Concerns with the settling of the fuel bed so that combustion takes place without the need to poke or otherwise stir the fuel bed have provoked a variety of vessel construction. None of these lends themselves well to a high volume, precisely controlled, continuous process wherein the biomass fuel is efficiently converted to the target gas for supply to and likely, additional energy or waste in the process. Exposing the base fuel during the pyrolysis to air, water vapor or other components has a direct impact on the products of pyrolysis, as does the temperature of the process and the duration thereof. By using any of the processes of the prior art, such as a fluidized bed, which is, at least initially exposed to air and can be additionally exposed to oxygen, or other input gases, some portion of the fuel for gasification is consumed, as by oxidation (burning) affecting the output of the process by producing ash or other undesirable residue.

Although several prior processes have met with varying degrees of both commercial and technical success, there is still a need in the art for improved and more efficient processes for converting biomass to synthetic natural gas.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a process for converting carbonaceous material to synthetic natural gas, which process comprising:

a) passing said carbonaceous material and an effective amount of superheated steam to a reforming zone operated at a temperature of about 850° C. to about 1200° C. and a pressure form about 3 psig to about 500 psig wherein said carbonaceous material is reformed to produce a synthetic gaseous product comprised of hydrogen, carbon monoxide, carbon dioxide, and methane, which synthetic gaseous product stream is at an elevated temperature;

b) passing said synthetic gaseous product stream at an elevated temperature to a heat recovery zone wherein its temperature is substantially lowered;

c) passing said lowered temperature synthetic gaseous product stream to a solids recovery zone wherein substantially all remaining solids are removed;

d) passing said synthetic gaseous product stream from said solids recovery zone to an organics removal zone wherein substantially all of any organic material is removed by contact with an organic liquid in which the organic material is substantially soluble;

e) passing said synthetic gaseous product stream from said organics removal zone to an acid gas removal zone wherein substantially all acid gases are removed by use of an acid gas;

f) passing said synthetic gaseous product stream from said acid gas removal zone to a methanation process unit containing at least one methanation catalyst and operated at methanation process conditions thereby resulting in a product stream comprised predominantly of methane.

In a preferred embodiment there is a water wash step between before the organic removal step wherein the synthetic gaseous product stream is passed countercurrent to a stream of water to remove any remaining solids.

In another preferred embodiment the carbonaceous material is selected from the group consisting of wood and dried distillers grains.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 hereof is a generalized flow scheme of a preferred embodiment of the present invention wherein a carbonaceous material, such as wood chips, are steam reformed to produce a syngas, which is then passed through various clean-up steps to a methanation unit to produce synthetic natural gas.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to the production of synthetic natural gas (predominantly methane) from carbonaceous materials, preferably biomass materials. Synthetic natural gas, also sometimes called “green gas” is a renewable gas from biomass with natural gas specifications. Therefore, it can be transported through the existing gas infrastructure, substituting for natural gas in all existing applications. Another advantage of green gas is that is carbon neutral. That is, using biomass as an energy supply will typically not result in a net CO₂ emission since its source can be replanted and uses CO₂ from the atmosphere during its growth period.

While this invention is applicable to a broad range of carbonaceous feedstocks including the traditional naturally occurring solid fossil fuels such as coal, peat, lignite, tar sands,and bitumen from oil shale, the preferred feedstocks for use in the present invention are biomass feedstocks Non-limiting examples of biomass feedstocks suitable for being converted in accordance with the present invention include trees such as red cedar, southern pine, hardwoods such as oak, cedar, maple and ash, as well as bagasse, rice hulls, rice straw, kennaf, old railroad ties, dried distiller grains, corn stalks and cobs and straw. Cellulosic materials are the more preferred biomass feedstocks, with wood and dried distillers grains being the most preferred. Biomass is typically comprised of three major components: cellulose, hemicellulose and lignin. Cellulose is a straight and relatively stiff molecule with a polymerization degree of approximately 10,000 glucose units (C₆ sugar). Hemicellulose are polymers built of C₅ and C₆ sugars with a polymerization degree of about 200 glucose units. Both cellulose and hemicellulose can be vaporized with negligible char formation at temperatures above about 500° C. On the other hand, lignin is a three dimensional branched polymer composed of phenolic units. Due to the aromatic content of lignin, it degrades slowly on heating and contributes to a major fraction of undesirable char formation. In addition to the major cell wall composition of cellulose, hemicellulose and lignin, biomass often contains varying amounts of species called “extractives”. These extractives, which are soluble in polar or non-polar solvents, are comprised of terpenes, fatty acids, aromatic compounds and volatile oil.

In most instances the carbonaceous feedstock used in the practice of the present invention will be found in a form wherein the particles too large for conducting through the tubes of the reformer. Thus, it will usually be necessary to grind the carbonaceous material to an effective size. In this case, the carbonaceous material is ground, or otherwise reduced in size, to a suitable size of about 1/32 inch to about 1 inch, preferably about 1/16 inch to about ½ inch, and more preferably from about ⅛ inch to ¼ inch. Grinding techniques are well know and varied, thus any suitable grinding technique and equipment can be used for the particular carbonaceous material being converted.

Generally, reforming requires that a feedstock have less than about 15% moisture content, but there is an optimization between moisture content and conversion process efficiency. The actual moisture content will vary somewhat depending on the commercial process equipment used. Since some of the biomass received for processing can have a moisture content from about 40 to 60% it will have to be dried before pyrolysis. Any conventional drying technique can be used as long as the moisture content is lowered to less than about 15% when mixed with the superheated steam. For example, passive drying during summer storage can reduce the moisture content to about 30% or less. Active silo drying can reduce the moisture content down to about 12%. Drying can be accomplished either by very simple means, such as near ambient, solar drying or by waste heat flows or by specifically designed dryers operated on location. Also, commercial dryers are available in many forms and most common are rotary kilns and shallow fluidized bed dryers.

This invention can be better understood with reference to the sole figure hereof. The carbonaceous material feedstock is conducted via line 10 and superheated steam is conducted via line 12 to mixing zone Mix wherein the two are sufficiently mixed before being conducted via line 14 into steam reformer R. The superheated steam, which will be at a temperature from about 850° F. to about 950° F. acts as both a source of hydrogen as well as a transport medium. When mixed with the carbonaceous material the resulting mixture must be kept above its dew point before entering the reforming. The dew point will typically be at about 230° C. The amount of superheated steam to feedstock will be an effective amount. By effective amount we mean at least that amount needed to provide sufficient transport of the feedstock. That ratio of superheated to steam of feedstock, on a volume to volume basis will typically from about 0.2 to 2.5, preferably from about 0.3 to 1.0. The temperature conditions for the pyrolysis reaction will be described later in detail. The steam is preferably introduced so that the feedstock is diluted to the point where it can easily be transported through the reactor tubes. Fluidization will typically result and can realize fluid pyrolysis by virtue of good contact among steam, polymers and heat decomposition products of carbonaceous material liberated in the gas phase.

The mixture of steam and feedstock is fed to steam reformer R via line 14 into a flow divider FD where it is distributed into the plurality of coiled reactor tubes of effective internal diameter and length within a metal cylindrical vessel of suitable size. Typical internal diameters for the pyrolysis reactor tubes will be from about 2 to about 4 inches, preferably from about 2.5 to about 3.5 inches, and more preferably about 3 inches.

Although the source of heat for the reformer can be any suitable source it is preferred that the source of heat be one or more burners B located at bottom of the reforming process unit. Fuel for burner B can be any suitable fuel. It is preferred that at least some of the fuel be obtained from the present process, such as fuel or syngas produced in the reformer.

It is preferred that reformer R be one in which the carbonaceous feed will be distributed to a plurality of vertically oriented tubes. At least a portion of the carbonaceous feed is converted to syngas in reformer R, which syngas is also composed primarily of hydrogen, carbon dioxide, carbon monoxide and methane. The inlet temperature of the feedstock and superheated steam entering reformer R will preferably be about 230° C. The exit temperature of the product syngas leaving reformer R via line 16 will typically be from about 850° C. and 1200° C., preferably between about 900° C. and about 1,000° C. At a temperature of about 1100° C. and above and with a contact time of about 5 seconds, one obtains less than about one mole percent of methane and large amounts of CO₂, which is an undesirable result. Pressure in the reformer is not critical, but it will typically be at about 3 to 35 psig. Also, it is preferred that the residence time in the reformer be from about 0.4 to about 1.5 seconds.

For any given feedstock, one can vary the proportions of hydrogen, carbon dioxide, carbon monoxide and methane that comprise the resulting syngas product stream as a function of the contact time of the carbonaceous feedstock in the reformer, the exit temperature, the amount of steam introduced, and to a lesser extent, pressure. Certain proportions of syngas components are better than others for producing synthetic natural gas, thus conditions should be such as to maximize the production of carbon monoxide and methane at the expense of hydrogen.

Returning now to the Figure hereof a flue gas stream comprised primarily of CO₂ and N₂ is exhausted from the reformer via line 15 and the product syngas stream from reformer R is conducted via line 16 to heat recovery zone HR1 where it is preferred that water be the heat exchange medium and that the water be used as preheated steam to reformer R via lines 18 where it is further heated to produce at least a portion of the superheated steam used for the reformer via line 17. Heat Recovery zone HR1 can be any suitable heat exchange device, such as the shell-and-tube type wherein water is used to remove heat from product stream 16. From heat recovery zone HR1 the product syngas is passed via line 18 through separation zone S which contains a gas filtering means and preferably a cyclone (not shown) and optionally a bag house (not shown) to remove at least a portion, preferably substantially all, of the remaining ash and other solid fines from the syngas. The filtered solids are collected via line 20 for disposal.

The filtered syngas stream is then passed via line 22 to water wash zone WW wherein it is conducted upward and countercurrent to down-flowing water via line 23. The water wash zone preferably comprises a column packed with conventional packing material, such as copper tubing, pall rings, metal mesh or other such materials. The syngas passes upward countercurrent to down-flowing water which serves to further cool the syngas stream to about ambient temperature, and to remove any remaining ash that may not have been removed in second separation zone S. The water washed syngas stream is then passed via line 24 to oil wash zone OW where it is passed countercurrent to a downflowing organic liquid stream to remove any organics present, such as benzene, toluene, xylene, or heavier hydrocarbon components via line 25 that may have been produced in the reformer. The downflowing organic stream will be any organic stream in which the organic material being removed is substantially soluble. It is preferred that the down-flowing organic stream be a hydrocarbon stream, more preferably a petroleum fraction. The preferred petroleum fractions are those boiling in naphtha to distillate boiling range, more preferably a C₁₆ to C₂₀ hydrocarbon stream, most preferably a C₁₈ hydrocarbon stream.

The resulting syngas stream is conducted via line 26 to acid gas scrubbing zone AGS wherein acidic gases, preferably CO₂ are removed. Any suitable acid gas treating technology can be used in the practice of the present invention. Also, any suitable acid gas scrubbing agent can be used, preferably a basic solution can be used in the acid gas scrubbing zone AGS that will adsorb the desired level of acid gases from the vapor stream. It will be understood that it may be desirable to leave a certain amount of CO₂ in the scrubbed stream depending on the intended use of resulting methane product stream from the methanation unit. For example, if the methane product stream is to be introduced into a natural gas pipeline, no more than about 4 vol. % of CO₂ should be remain. If the methane product stream is to be used for the production of methanol, then at least that stoichiometric amount of CO₂ needed to result in the production of methanol should remaing. One suitable acid gas scrubbing technology is the use of an amine scrubber. Non-limiting examples of such basic solutions are the amines, preferably diethanol amine, mono-ethanol amine, and the like. More preferred is diethanol amine. Another preferred acid gas scrubbing technology is the so-called “Rectisol Wash” which uses an organic solvent, typically methanol, at subzero temperatures. The scrubbed stream can also be passed through one or more guard beds (not shown) to remove catalyst poisoning impurities such as sulfur, halides etc. The treated stream is passed via line 28 from acid gas scrubbing zone AGS to methanation zone M. Methanation of syngas involves a reaction between carbon oxides, i.e. carbon monoxide and carbon dioxide, and hydrogen in the syngas to produce methane and water, as follows:

CO+3H₂

CH₄+H₂O   (1)

CO₂+4H₂

CH₄+2H₂O   (2)

Methanation reactions (1) and (2) take place at around 300° C. to about 900° C. in methanation zone M which is preferably comprised of two or more, more preferably three, reactors each containing a suitable methanation catalyst. The methanation reaction is strongly exothermic. Generally, the temperature increase in a typical methanator gas composition is about 74° C. for each 1% of carbon monoxide converted and 60° C. for each 1% carbon dioxide converted. Because of the exothermic nature of methanation reactions (1) and (2), the temperature in the methanation reactor during methanation of syngas has to be controlled to prevent overheating of the reactor catalyst. Also high temperatures are undesirable from an equilibrium standpoint and reduce the amount of conversion of syngas to methane since methane formation is favored at lower temperatures. Formation of soot on the catalyst is also a concern and may require the addition of water to the syngas feedstock.

A preferred way to control heat during the methanation reaction is use a plurality of reactors with heat removed between each reactor. Thus, methanation zone M preferably comprises a series of three adiabatic methanation reactors R1, R2 and R3. Each of these reactors is configured to react carbon oxide and hydrogen contained in the syngas in the presence of a suitable catalyst to produce methane and water, in accordance with the reactions (1) and (2) set forth hereinabove. Each of the methanation reactors includes a catalyst capable of promoting methanation reactions between carbon oxides and hydrogen in the syngas feedstock. Any conventional methanation catalyst is suitable for use in the practice of the present invention, although nickel catalysts are most commonly used and the more preferred for this invention. Such catalysts are, especially those containing greater than 50% nickel, are generally stable against thermal and chemical sintering during methanation of undiluted syngas streams. Alternatively, other stable catalysts that are active and selective towards methane may be used in the methanation reactors.

As previously mentioned because the methanation reaction is strongly exothermic, heat needs to be removed between reactors. Thus, heat recover zones HR2 and HR3 are used to remove heat from the stream as it passed from reactor R1 to reactor R2 and reactor R2 to reactor R3 respectively. Any suitable exchange device can be used, preferably a shell-and-tube type wherein water can be used to remove heat from the product stream. The water can then be recycled to line 30 where it can be further heated to produce superheated steam. As can be appreciated from the above and as shown in the examples discussed below, the inlet and outlet temperatures of the streams entering and exiting methanation reactors R1-R3 can be controlled by varying the percentage of syngas being delivered to each of the reactors as well as how much heat is exchanged by heat exchangers HR2 and HR3. Typically, the inlet temperature of reactors R1 and R2 will be from about 400° F. to about 450° F. with an outlet temperature of about 500° F. to about 800° F. The third reactor, which will operate at a lower temperature than that of reactors R1 and R2 will have an inlet temperature of about 400° F. and an outlet temperature of about 500° F.

In a preferred embodiment of the present invention, the step of recovering at least a part of generated heat and/or at least a part of waste heat in the regeneration zone and effectively utilizing the recovered heat is further provided. The recovered heat can be effectively utilized, for example, for drying and heating of the biomass feedstock and the generation of steam as the gasifying agent.

The product stream from the methanation unit will be comprised predominantly of methane. That is, it will contain at least about 75 vol. %, preferably at least about 85 vol. %, and more preferably at least about 95 vol. % methane. The product methane can be introduced into a natural gas pipeline and utilized at any downstream facility. One such facility if preferably a plant that converts the methane to syngas then to other products, such as alcohols, transportation fuels, or lubricant base stocks. If it is desired to produce syngas from the methane produced in the methanation unit M, then any suitable process can be used that convert methane or natural gas to syngas. Preferred methods include steam reforming and partial oxidation. More preferred is steam reforming. Steam reforming of methane is a highly endothermic process and involves following reactions:

Main reaction

CO₂ reforming of methane: It is also a highly endothermic process and involves the following reactions:

Main reaction

The steam reformer will preferably be one similar to reformer R hereof, which is a coiled tubular reactor. Preferred steam reforming catalysts are nickel containing catalysts, particularly nickel (with or without other elements) supported on alumina or other refractory materials, in the above catalytic processes for conversion of methane (or natural gas) to syngas is also well known in the prior art. Kirk and Othmer, Encyclopedia of Chemical Technology, 3rd Ed., 1990, vol. 12, p. 951; Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., 1989, vol. A12, pp. 186 and 202; U.S. Pat. No. 2,942,958 (1960); U.S. Pat. No. 4,877,550 (1989); U.S. Pat. No. 4,888,131(1989); EP 0 084 273 A2 (1983); EP 0 303 438 A2 (1989); and Dissanayske et al., Journal of Catalysis, vol. 132, p. 117 (1991).

The catalytic steam reforming of methane, or natural gas, to syngas is a well established technology practiced for commercial production of hydrogen, carbon monoxide and syngas (i.e., a mixture of hydrogen and carbon monoxide). In this process, hydrocarbon feed is converted to a mixture of H₂, CO and CO₂ by reacting hydrocarbons with steam over a supported nickel catalyst such as NiO supported on alumina at elevated temperature (850° C. to 1000° C.) and pressure (10-40 atm) and at steam to carbon mole ratio of 2-5 and gas hourly space velocity of about 5000-8000 per hour.

This process is highly endothermic and hence it is carried out in a number of parallel tubes packed with a catalyst and externally heated by flue gas to a temperature of 980° C. to about 1040° C. (Kirk and Othmer, Encyclopedia of chemical Technology, 3rd, Ed., 1990, vol. 12, p. 951, Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., 1989, vol. A12, p. 186). 

1. A process for converting carbonaceous material to synthetic natural gas, which process comprising: a) passing said carbonaceous material and an effective amount of superheated steam to a reforming zone operated at a temperature of about 850° C. to about 1200° C. and a pressure form about 3 psig to about 500 psig wherein said carbonaceous material is reformed to produce a synthetic gaseous product comprised of hydrogen, carbon monoxide, carbon dioxide, and methane, which synthetic gaseous product stream is at an elevated temperature; b) passing said synthetic gaseous product stream at an elevated temperature to a heat recovery zone wherein its temperature is substantially lowered; c) passing said lowered temperature synthetic gaseous product stream to a solids recovery zone wherein substantially all remaining solids are removed; d) passing said synthetic gaseous product stream from said solids recovery zone to an organics removal zone wherein substantially all of any organic material is removed by contact with an organic liquid in which the organic material is substantially soluble; e) passing said synthetic gaseous product stream from said organics removal zone to an acid gas removal zone wherein substantially all acid gases are removed; f) passing said synthetic gaseous product stream from said acid gas removal zone to a methanation process unit containing at least one methanation catalyst and operated at methanation process conditions thereby resulting in a product stream comprised predominantly of methane.
 2. The process of claim 1 wherein the carbonaceous material is a source of fossil fuels selected from the group consisting of coal, peat, lignite, tar sands,and bitumen from oil shale.
 3. The process of claim 1 wherein the carbonaceous material is a biomass material.
 4. The process of claim 3 wherein the biomass material is a cellulosic material.
 5. The process of claim 4 wherein the cellulosic material is selected from the group consisting of wood, bagasse, rice hulls, rice straw, kennaf, old railroad ties, dried distiller grains, corn stalks and cobs and straw.
 6. The process of claim 5 wherein the cellulosic material is selected from wood and dried distiller grains.
 7. The process of claim 1 wherein the carbonaceous material is dried to a moisture content of less than or equal to about 15% by weight before reforming.
 8. The process of claim 1 wherein the carbonaceous material is with the size range of about 1/32 inch to about ½ inch.
 9. The process of claim 1 wherein the heat recovery zone uses water to recover heat and wherein at least a portion of the heated water is used as preheated steam to the reforming process unit.
 10. The process of claim 1 wherein the acid gas scrubbing agent used in the acid gas removal zone is selected from the group consisting of amines and alcohols.
 11. The process of claim 10 wherein the acid gas scrubbing agent is an alcohol.
 12. The process of claim 11 wherein the alcohol is methanol.
 13. The process of claim 10 wherein the acid gas scrubbing agent is an amine selected from the group consisting of diethanol amine and mono-ethanol amine.
 14. The process of claim 13 wherein the amine is diethanol amine.
 15. The process of claim 1 wherein the methanation zone contains three reactors in series and wherein heat is removed from the stream passing from the first reactor and the second reactor.
 16. The process of claim 1 wherein at least a portion of the methane produced in the methanation unit is introduced into a natural gas pipeline.
 17. The process of claim 14 wherein methane is removed from a pipeline and converted to a syngas.
 18. The process of claim 1 wherein prior to organic removal step (j) the synthetic gaseous product stream is subjected to a water wash wherein is flowed countercurrent to a stream of water to remove any remaining solids material. 